In certain weather conditions or poor road conditions it is possible for a vehicle to become stuck, for example in a depression or on slippery subsoil, so that the stuck position can no longer be left by driving off in a conventional manner. By starting off toward the obstacle, then disengaging the clutch, allowing the vehicle to roll back, and then repeating the procedure, a driver with a manual-shift vehicle can build up momentum and release the vehicle. This process is known a rocking free. For vehicles having an automatic transmission, an automated transmission or a multiple-clutch transmission the method described above can only be used in some circumstances. Such vehicles are therefore often provided with an automatic method for rocking free, which enables the vehicle to be released from its stuck position.
The rocking-free process is either initiated by a driver, or automatically started in accordance with previously stored criteria such as a large difference between the speeds of the individual wheels. The driver can often activate the automatic initiation of the rocking-free process by means of a switch or by actuating a key, so that the rocking-free process is only started when the driver so wishes. Once the vehicle has been released the rocking-free process is terminated, for example by actuating the switch or key again.
DE 10 2004 017 422 A1 describes a method for implementing a rocking-free function, which, in order to initiate the rocking-free process, evaluates the wheel rotational speeds of the driven wheels of a vehicle. From that, the traction capacity of the corresponding wheels is estimated. Furthermore, among other things, information about rotational direction and inclination is used in order to optimize the rocking-free process. The rocking-free function can either be activated when a driver of the vehicle actuates a switch or key, or by a command from the on-board computer. Likewise an actuation is disclosed, which after registering that the vehicle is stuck, proceeds automatically or after questioning the driver. The rocking-free process is terminated when the driver actuates the switch or key, or automatically if vehicle-internal systems register that the vehicle is moving in one direction or over a fixed distance for a longer time. It is also disclosed that in vehicles having a distance-warning system, it is tested whether sufficient free areas are around the vehicle concerned, to carry out the rocking free. If this is not the case, the rocking-free process is suppressed.
DE 101 28 853 A1 describes a method for rocking free and/or maneuvering a vehicle with a multiple-clutch transmission, in which the rocking-free process can only be started when the clutch arrangement is in a disengaged condition and/or the vehicle concerned is at rest. In addition it is disclosed that this special transmission condition can be produced by a driver of the vehicle by means of an operating arrangement which, for example, can consist of two switches or an operating lever. The vehicle's shift lever can also serve as the operating lever. The rocking-free process is initiated when the wheel-slip of the vehicle's driven wheels or the force acting in opposition to the drive torque of the vehicle exceed a predetermined limit value or when the vehicle's speed falls below a set limit value. A rocking-free period ends when the vehicle comes to a standstill, or when the wheel-slip becomes too great.
DE 101 09 662 B4 describes a method for controlling a dual-clutch transmission, in which a direction-change mode, which is comparable to a rocking-free process, can indirectly be triggered by the driver. The direction-change mode ends automatically. As an interruption criterion the actuation of a brake pedal is used, which lasts for a predetermined time and beyond that predetermined time. Likewise the direction-change mode ends if a brake pedal actuation would lead to a predetermined brake pressure and/or to a predetermined braking force. Furthermore, the direction-change mode can also be terminated by actuating a drive pedal for up to or longer than a predetermined time, and/or which reaches or exceeds a predetermined limit value of the pedal movement. A termination of the direction-change mode due to the movement of the vehicle for a predetermined time in one direction and/or over a predetermined distance is also disclosed.
Furthermore, vehicles with an automated transmission are also known, which have a rocking-free function. An implemented rocking-free process is activated by a driver by means of a key and the simultaneous engagement of the first or second reverse gear, or by engaging the first to the eighth forward gear while at the same time driving more slowly than a speed of 5 km/h. The rocking-free function is deactivated either if the driver actuates the key again, or by driving at a speed above or equal to 8 km/h.
For example, if a vehicle has become stuck on muddy ground and a rocking-free function is started in order to release the vehicle, it can happen after a successful release of the vehicle that the rocking-free function is not terminated. For example, the driver of the vehicle can forget to switch off the rocking-free function by actuating the switch or key. The result is that each time the vehicle starts off a rocking-free function is initiated. This makes starting uncomfortable for the driver and can lead to accidents. The same situation arises when speed or distance limits are chosen too high or determined erroneously. It is therefore appropriate to establish further criteria that bring about a suppression of the rocking-free process, so that the rocking-free function is carried out safely by virtue of the automatic suppression.